The present invention relates to an LiGaO.sub.2 single crystal, an LiGaO.sub.2 single-crystal substrate, and a method of manufacturing the same.
In recent years, III-V nitride compound semiconductors (Ga, In, Al)N have received a great deal of attention as one of light-emitting device material candidates in a short wavelength range of blue or bluish green to ultraviolet.
These nitride-based materials are characterized in their high hardness, high melting points, and high thermal conductivities. In addition, they have a direct transition type band structure as an energy band structure. The bandgap energy at room temperature can be changed from 1.95 eV to 6.0 eV using a mixed crystal of the nitride-based materials. The nitride-based materials are expected to be applied not only as light-emitting devices but also high-power transistors or environmental resistance semiconductor devices because of their large band gaps.
Conventionally, no appropriate substrate for epitaxially growing a gallium-nitride-based semiconductor is available, and a gallium-nitride-based semiconductor thin film is grown on a sapphire substrate (Al.sub.2 O.sub.3) substrate regardless of the large lattice mismatch (13%) between GaN film and sapphire substrate. However, when GaN is directly grown on the sapphire substrate at a high temperature, GaN exhibits a three-dimensional hexagonal pyramidal growth pattern because of the large lattice mismatch, so the crystal growth surface cannot be a flat mirror surface.
For this reason, AlN or GaN is generally grown at a low temperature as an amorphous film serving as a buffer layer, and GaN is grown on the buffer layer at a high temperature.
However, the dislocation density of the GaN film grown by this method is as high as 10.sup.8 to 10.sup.9 cm.sup.-2, so the film cannot be regarded as a high-quality epitaxially grown film, posing a serious problem in practical use of optical or electron devices.
From this viewpoint, to lower the dislocation density of an epitaxially grown GaN film, strong demand has arisen for a substrate material having smaller lattice mismatch to GaN.
As a substrate material having smaller lattice mismatch to GaN than sapphire, 6H-SiC is most popularly used. However, 6H-SiC has a high melting point (2,700.degree. C.), which makes it difficult to manufacture a large single crystal. In addition, a defect called a micropipe is readily formed in growing the single crystal, so the crystal quality as a substrate material is unsatisfactory.
Unlike the conventional substrate materials, lithium gallate (LiGaO.sub.2) has a distorted wurtzite type crystal structure, i.e., almost the same crystal structure as that of GaN as a hexagonal crystal. The lattice mismatch to GaN as the hexagonal crystal is estimated as about 0.9% on the basis of the lattice constant of LiGaO.sub.2, i.e., much smaller than that of the conventional substrate material. A GaN thin film can be grown on the c-plane of the LiGaO.sub.2 single-crystal substrate.
Conventionally, however, the following problem is posed. The LiGaO.sub.2 crystal is an inverted symmetrical crystal belonging to point group mm2, i.e., space group Pna2.sub.1 and has a polarity along the c-axis direction. When this crystal is pulled up in the c-axis direction by the Czochralski (CZ) method, and a c-plane substrate is formed by cutting the c-plane perpendicular to the c-axis direction, -c and +c regions having a domain structure are simultaneously formed on the substrate surface because of polarity inversion.
Even when the GaN thin film is grown, a region where the thin film is epitaxially grown and a region where the thin film is peeled off are formed, so the LiGaO.sub.2 crystal cannot be used as the substrate for epitaxial growth in practice.
When the -c and +c regions are simultaneously present as the domain structure, the substrate surface has a step as will be described below.
The c-plane perpendicular to the c-axis direction is cut from the LiGaO.sub.2 single crystal which is formed by pulling up the crystal in the c-axis direction by the CZ method, thereby forming a single-crystal substrate. When the surface of the single-crystal substrate is polished by chemical-mechanical polishing, a crossed line 202 radially extending from the center of a substrate 201 is observed, as shown in FIG. 2A.
More detailed observation of the crossed line portion reveals that +c regions and -c regions alternate, as shown in FIG. 2B. FIG. 2B shows a section taken along an arrow in FIG. 2A.